Yes, it's an authentic & very beneficial book authored by the grandson of the Mujaddid of Hind, 'Allaamah Shah Waliullaah Muhaadith Dehlawee (rahimahullaah) . It is a very good book on Tawheed & a goodo ne to give Da'wah to the Barelwees & Deobandees, since they recognise the authority of the both the Shaykhs.

In this book, Shah Ismail Shaheed has gathered under different topics all the horrible blunders of Faith and practice which are contrary to the teachings of Islamic Monotheism, for instance, committing an act of Shirk (associating partners to Allaah) in terms of knowledge, regulating the affairs of the universe, habits and practices, and shirk in worshipping. Hence Taqwiyat-ul-Imaan has become an authentic and unique book on the subject of Tawheed.

Shah Isma'il al-dahlawi is the grandson of waliyyullaah al-dahlawi. Shaheed is an honorific title and not part of his name it is appended to his name due to him dieing in battle. As for the book taqwiyatul-eemaan the strengthening of faith

The book comprises of a clarification of tawheed and a refutation of numerous polytheistic practices common to the Indian sub-continent. And it is worthy of mention that this book has shaken shirk and bid'ah in the Indian sub continent (and for this is hated by the bareilvees and those like them).This book is very beneficial in clarifying tawheed and refuting shirk and resembles The book of Tawheed written by Muhammad ibn Abdulwahhaab